Well actually a reef tank needs stability. A 3 gal
will be more difficult in this respect, and I wouldn't even recommend a fish in such a small tank, and any evaporation will cause the salinity to rise. So I think a 10 gallon should be the minimum here. You will need some good lighting I would recommend PC
, and start with soft corals such as zoanthids, palys, mushrooms, xenia, green star polyps. Then you need to keep proper calcium levels, which weekly water changes should supply with no problem. I would suggest you get some more research done on reef tanks, and keeping corals.